This disclosure relates to a calibration device for a carbon dioxide sensor. In particular, this disclosure related to internal generation of a carbon dioxide reference gas to calibrate a carbon dioxide sensor.
Instruments and transducers for the measurement of carbon dioxide (CO2) concentration in air have been described in the patent literature, and also in the form of commercially available products on the market. Many of these instruments are based on the absorption spectra within the infrared wavelength area of electromagnetic radiation of the CO2 molecule. Such spectra can be detected and analyzed by spectroscopic instruments according to known technology. By measuring at specific wavelengths where the absorption of CO2 deviates from other constituents of air, it is possible to extract an output signal with required sensitivity and specificity.
Other instruments are based on solid electrolyte or an electrochemical cell that uses a catalytic electrode so that the carbon dioxide is either oxidized or reduced with the exchange of electrons. The flow of current due to the oxidation or reduction of the carbon dioxide is then detected as a measure of the concentration of the detected carbon dioxide. However, one problem with carbon dioxide sensors is that they lose sensitivity over time. For example, the working life of an electrochemical cell is determined by the activity of the cell's catalytic electrode that is used to detect carbon dioxide within the sensor. This activity is gradually reduced over time by contaminant gases and poisons such that the sensitivity of the sensor drifts downward.
If the instrument into which the carbon dioxide sensor is built is calibrated regularly, this downward sensitivity drift can be compensated for by adjusting the gain of the carbon dioxide sensor, and any faulty carbon dioxide sensors can be replaced immediately. However, if the instrument is in a difficult position to service, or if calibration of the carbon dioxide sensor is not otherwise freely available, it is often impossible to confirm that the carbon dioxide sensor is functioning correctly. Therefore, as the carbon dioxide sensor reaches the end of its working life, the output of the sensing cell may be insufficient to generate an alarm or other condition. As a result, a situation could arise where toxic levels of gas are present, but the carbon dioxide sensor is incapable of providing the requisite warning.